The present invention relates to a process for preparing pre-expanded particles of a thermoplastic resin. More particularly, the invention relates to a process for preparing pre-expanded thermoplastic resin particles which are applicable, without further imparting expandability to the pre-expanded particles prior to molding, to a process for producing foams by heating pre-expanded particles with a heating medium such as steam in a mold which is able to be closed but unable to be sealed, in other words, in a mold which is able to be closed but is permeable to a fluid heating medium.
As a process for pre-expanding particles of a thermoplastic resin, there are, for example, a process disclosed in DE-OS No. 2,107,683 and a process disclosed in Japanese Examined patent publication No. 56-1344, and the like. According to the processes, pre-expanded particles of a thermoplastic resin are obtained and they can be used for molding in a mold.
The cell diameter of the pre-expanded thermoplastic resin particles obtained by means of these processes is, however, usually 200 .mu.m or less, and it is difficult to control the cell diameter of the particles. Japanese Unexamined patent publication No. 58-199125 discloses decreasing the diameter of cells of pre-expanded particles by using an inorganic fine powder such as talc or silica, but no technique for incresasing the diameter of cells has been known.
In case that the cell diameter of pre-expanded particles is too small, it is necessary, prior to molding the pre-expanded particles, to further impart expandability to the pre-expanded particles (Japanese Unexamined patent publication No. 59-187036) or to conduct aging the particles under pressure (Japanese Examined patent publication No. 60-10047). Conducting such operations makes the process complicated and causes economic disadvantage. Further, even when the above operations are conducted, there remains a problem that the range of allowable molding condition is still very narrow.
An object of the present invention is to provide a process for preparing pre-expanded particles of a thermoplastic resin, by which pre-expanded particles having a large cell diameter can be obtained.
This and other objects of the present invention will become apparent from the description hereinafter.